The coupling efficiency of diode lasers to optical fibers is theoretically investigated at 1.55 micrometers wavelength. A waveguide device in glass for low-loss multiplexing of diode laser arrays to optical fibers is proposed. The device is composed of cascaded Y-junctions made by single ion exchange in glass with taper waveguide at input or output end. The taper waveguide is made by a double ion exchange process. Theoretical simulations indicate that approximately 5 dB improvement per channel, compared to a simple multiplexer, in coupling of diode lasers to single mode optical fibers can be achieved using the proposed device.